A specific structure for a conventional positron emission tomography apparatus (PET), for imaging the distribution of a radiopharmaceutical, will be explained. A conventional PET apparatus is provided with a detector ring wherein radiation detectors, which detect radiation, are arranged in a circular ring shape. This detector ring detects pairs of radiation (known as “annihilation radiation pairs”) that are emitted in opposite directions from a radiopharmaceutical within the body of a subject.
The structure of a radiation detector 51 will be explained. The radiation detector 51, as illustrated in FIG. 17, comprises scintillators 52 that are arranged three-dimensionally, and a photodetector 53 for detecting fluorescent emission produced from the radiation absorbed by the scintillators 52. The photodetector 53 is provided with a detecting surface wherein a large number of photodetecting elements are arranged in a matrix. Given this, the detecting face of the photodetector 53 and one face of the scintillator 52 are coupled optically.
When radiation is incident on a scintillator 52, a fluorescent emission is produced within the scintillator 52. This fluorescent emission requires some time in order to decay completely. As a result, the scintillator 52 will continue to emit light weakly for some time when there is incident radiation.
One method for measuring such a fluorescent emission is a method wherein the intensity of the fluorescent emission is sampled at regular time intervals. Because such a method has the simplicity of detecting the fluorescent emission discontinuously, there is a benefit in that the structure of the circuit for the radiation detector 51 can be simplified to some degree.
The top section in FIG. 18 is for explaining a conventional method for measuring a fluorescent emission. When a fluorescent emission is produced in the scintillator 52, the intensity of the fluorescent emission exhibits a sharply rising edge, after which it gradually diminishes. The radiation detector 51 measures the instantaneous intensities of the fluorescent emission at the times indicated by the dotted line at the top in FIG. 18. Given this, at the time of evaluating the degree of intensity of the fluorescent emission, having some duration in time, in the scintillator 52, a summation value is used wherein fluorescent intensities obtained at different moments are added together. The greater the summation value, the more intense the fluorescent emission produced can be assumed to be.
The number of instantaneous fluorescent intensities that are to be added together when calculating the summation value is determined in advance in the fluorescent emission detector. In the example at the bottom in FIG. 18, the structure is such that fluorescent intensities from 7 points in time are summed together to arrive at the summation value (referencing, for example, Patent Document 1).